5624
J . Org. Chem. 2001, 66, 5624-5626
Sch em e 1
Selective Red u ction of Ter m in a l Alk yn es to
Alk en es by In d iu m Meta l
Brindaban C. Ranu,*,† J yotirmoy Dutta, and
Sankar K. Guchhait
Department of Organic Chemistry,
Indian Association for the Cultivation of Science,
J adavpur, Calcutta - 700 032, India
procedure to produce the corresponding allyl compounds
in high yields. The results are presented in Table 1. As
shown in Table 1, although efficient reductions were
achieved in phenyl or substituted phenyl ethers, amines,
or esters within a reasonable period, benzyl (entry 9),
phenyl propyl (entry 10), and other alkyl derivaties
(entries 11, 19, 23) were found to be reluctant to undergo
reductions. Nonfunctionalized (entry 24) and internal al-
kynes (entry 25) remained completely inert. On the other
hand, phenyl propargyl thioether (entry 26) could not
tolerate the reaction condition and led to a mixture of
decomposition products within 1 h. However, in general
the reactions are very clean and no product other than
the reduced one and the starting material were isolated.
Most significantly, no over-reduction beyond alkene
was observed in any propargyl ether, amine, or ester
under this procedure by indium metal. Moreover, the
allyl moiety present in several alkynes (entries 4, 17, 21)
remained unaffected. A number of other readily reducible
functional groups such as CHO (entry 5), CdO (entry 6),
OBn (entry 7), OAc (entry 8), OTs (entry 16), CO2Me
(entry 15), and Cl (entry 22) also did not undergo any
change under the present reaction conditions. This is of
much synthetic importance as many reducing agents
such as LiAlH4, diborane, and Lindlar’s catalyst, which
are usually employed for reduction of acetylenes, are not
compatible with many of these functionalities. Although
the exact mechanism of such a selective reduction of
triple bond to double bond by indium metal is not yet
clear to us, it is evident from the results that the presence
of an aryl moiety, the heteroatom (O and N), and a
terminal acetylenic functionality are essential for this
reduction to proceed. Probably, the process involves the
formation of radical ions, which could be stabilized by
such groups.
ocbcr@mahendra.iacs.res.in
Received March 9, 2001
The selective reduction of alkynes to alkenes is a
challenging problem in organic synthesis. Although a
number of reagents such as Lindlar’s catalyst, sodium
in liquid ammonia, and lithium aluminum hydride are
commonly used for this purpose,1 these reductions are
not always clean in multifunctional alkynes due to
competing reduction of the double bond formed and other
reducible groups present in the molecule. Recently,
indium has emerged as one of the metals with high
synthetic potential.2 However, although indium has been
used extensively in carbonyl addition reactions,3 its use
in other domains has not been explored to any great
extent.2 Because of the close resemblance of indium to
magnesium and zinc in several respects including first
ionization potential, we envisioned indium to be a
potential reducing agent. Surprisingly, reduction by
indium is virtually unexplored except for a few recent
examples.4 This prompted us to initiate a systematic
investigation into the reduction by indium metal.4c,d
Although in general organoindium reagents are known
to be relatively inert toward unactivated carbon-carbon
double and triple bonds,2-4 we discovered that indium
metal reduces a terminal triple bond to a double bond in
aryl propargyl ethers, amines, and esters (Scheme 1).
In a typical experimental procedure, an aryl propargyl
ether, amine, or ester was heated under reflux with
pulverized indium (freshly made by sonication of indium
metal in ethanol) in aqueous ethanol for about 40 h.
Extraction with ether and usual workup provided the
corresponding allyl compound.
A wide range of structurally varied aryl propargyl
ethers, amines, and esters underwent reductions by this
To conclude, the present procedure using indium metal
in aqueous ethanol provides a highly selective reduction
of terminal alkynes to alkenes in aryl propargyl ethers,
amines, and esters. To the best of our knowledge, this is
the first report of reduction of an acetylenic triple bond
to an olefinic double bond by indium metal and certainly
it broadens the scope of indium-promoted reductions. The
significant features of this procedure are as follows: (a)
no undesired side reaction including over-reduction of the
double bond formed; (b) exclusive compatibility with
several easily reducible functionalities; (c) high yields;
and (d) green chemistry.
† Fax: 91-33-4732805.
(1) Hudlicky, M. Reductions in Organic Chemistry; Ellis Horwood
Limited: Chichester, 1984.
(2) (a) Li, C.-J . Chem. Rev. 1993, 93, 2023. (b) Cintas, P. Synlett
1995, 1087. (c) Li, C.-J . Tetrahedron 1996, 52, 5643. (d) Li, C.-J .; Chan,
T.-H. Tetrahedron 1999, 55, 11149. (e) Chauhan, K. K.; Frost, C. G. J .
Chem. Soc., Perkin Trans. 1 2000, 3015. (f) Ranu, B. C. Eur. J . Org.
Chem. 2000, 2347.
(3) (a) Paquette, L. A.; Kern, B. E.; Andino, J . M. Tetrahedron Lett.
1999, 40, 4129. (b) Melekhov, A.; Fallis, A. G. Tetrahedron Lett. 1999,
40, 7867. (c) Auge, J .; Germain, N. L.; Woaye, A. T. Tetrahedron Lett.
1999, 40, 9245. (d) Loh, T.-P.; Huang, J . M.; Xu, K.-C.; Goh, S. H.;
Vittal, J . J . Tetrahedron Lett. 2000, 41, 6511. (e) Nair, V.; J ayan, C.
N. Tetrahedron Lett. 2000, 41, 1091. (f) Khan, F. A.; Prabhudas, B.
Tetrahedron 2000, 56, 7595.
(4) (a) Moody, C. J .; Pitts, M. R. Synlett 1998, 1028. (b) Moody, C.
J .; Pitts, M. R. Synlett 1998, 1029. (c) Ranu, B. C.; Guchhait, S. K.;
Sarkar, A. Chem. Commun. 1998, 2113. (d) Ranu, B. C.; Dutta, P.;
Sarkar, A. J . Chem. Soc., Perkin Trans. 1 1999, 1139. (e) Reddy, G.
V.; Rao, G. V.; Iyenger, D. S. Tetrahedron Lett. 1999, 40, 3937. (f) Chae,
H.; Cho, S.; Keum. G.; Kang, S. B.; Pae, A. N.; Kim, Y. Tetrahedron
Lett. 2000, 41, 3899. (g) Yadav, J . S.; Reddy, B. V. S.; Reddy, M. M.
Tetrahedron Lett. 2000, 41, 2663.
Exp er im en ta l Section
Gen er a l Meth od s. 1H NMR (300 MHz) and 13C NMR (75
MHz) spectra were run in CDCl3 solutions. IR spectra were
taken as thin film. Analyses were done on a Perkin-Elmer 2400
autoanalyzer. Indium metal (Ingot) used was from SRL, India.
Gen er a l Exp er im en ta l P r oced u r e. Rep r esen ta tive P r o-
ced u r e for Red u ction of P h en yl P r op a r gyl Eth er . Phenyl
10.1021/jo010262z CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/04/2001